The present disclosure relates to solid-state lasers. More specifically, the present disclosure relates to a laser crystal having a tailored dopant profile, the method of fabricating thereof, and a lasing medium fabricated from said laser crystals.
Solid-state lasers are currently being developed and used for a variety of military and industrial applications, including range finding, target designation/marking, illumination, three-dimensional imaging, vibration sensing, profilometry, cutting, drilling, welding, heat treating and other material processing, electro-optical and infrared countermeasures, and directed energy weapons. A solid-state laser typically includes a laser amplifier medium or lasing medium disposed within an optical resonant cavity. The the resonant cavity or resonator provides the feedback necessary to build oscillation of electromagnetic radiation within the laser. The bulk lasing medium is typically in the shape of a slab, rod, or disk. When pumped, the medium provides amplification by a process of stimulated emission. The provision of reflective surfaces or gratings at the ends of the lasing medium provides a resonator.
In a typical laser, an incoherent light source imparts energy to the lasing medium, which produces light in which the waves are in phase through particular electron transitions. Where the lasing medium is properly designed, this “coherent light” is emitted as a beam.
Commercial laser gain media typically comprise single crystals having substantially uniform dopant concentration, such as Nd:YAG (neodymium doped yttrium-aluminum-garnet). Developmental lasers are being designed with optical-quality poly-crystalline ceramic lasing media which offer size and cost advantages over conventional single-crystal media. Solid-state lasing media, doped with an active ion, often use one or more flash lamps or laser diodes to provide “pump light.” The diode pump light excites the active ions in the doped crystalline or ceramic lasing medium to a higher energy state. This process is known as “absorption.” A “pump cavity” typically contains a uniformly doped lasing medium, which may be a crystal or glass or polycrystalline ceramic element fabricated in the shape of a rod, slab, or disk, and other elements, such as a pump light reflector or relay optics. Pump light is coupled into the cavity, typically with one or more flash lamps or laser diodes, either from the side of the cavity (i.e., side pumping) or the end of the cavity (i.e., end pumping).
Efficient absorption, in which nearly all of the pump light is absorbed by the doped medium, is a primary goal of laser designers. One method of attaining efficient absorption is by using high-absorption (highly doped) laser materials. A ray of pump light going through a doped crystal onetime is known as a “pass.” With most existing designs, a pump light ray makes only one or two passes through the doped crystal before escaping, necessitating the use of high-absorption materials to achieve efficient absorption. Absorption is governed by an exponential function. Thus, when such a crystal is side-pumped, non-uniform absorption and thus non-uniform gain often result, with the highest gain being near the edge of the lasing medium. The concentration of gain near the edge of the medium leads to problems with parasitic oscillation and amplified spontaneous emission (ASE), extraction, efficiency, and beam quality (mode control). This is particularly problematic with respect to rod shaped media.
Another approach to the goal of high efficiency absorption uses end pumping, in which pump light comes into a pump cavity along its longitudinal axis. End pumping requires high-brightness pump diodes and durable dichroic coatings, since the pumping and laser light extraction take place through the same optical surfaces (i.e. the ends of the rod) while requiring quite different reflectivity characteristics. In the case of quasi-four level (e.g., ytterbium doped yttrium aluminum garnet, Yb:YAG) or three-level systems (e.g., ruby) where the high threshold requires greater pumping rate, pump “bleaching” can occur, in which a large fraction of the active ions have been excited and correspondingly fewer ions are in the ground state available for pump light absorption, resulting in reduced absorption for both ‘Side- and end-pumping geometries.
A laser crystal having a tailored concentration profile may be especially useful for laser applications which may use a high-aspect-ration slab geometry for the lasing medium. One special case of the slab geometry is the planar waveguide (PWG) which is advantageous for applications requiring high gain, high average power, and high efficiency. As is known in the art, the PWG has a planar geometry, which guides light only in the thin dimension of the slab. For an end-pumped laser, the dopant absorbs the pump energy along the length of the medium and releases it radiatively as photons and non-radiatively as heat. Thus, heat is a function of both the energy pumped into the laser material and the dopant level of the laser material. Accordingly, as the pump energy and/or the dopant concentration are increased, both laser emission and heat generation are increased. Uniform dopant concentration that is typically used in lasing media results in localized heating. This is because the pump energy is absorbed by the dopant and thus decreases as it travels through the lasing medium. Accordingly, the material near the pump light end receives the most energy and produces the most heat, resulting in localized heating. Heat effects may have a negative impact on the laser efficiency and beam quality of high-average-power solid state lasers, and thus composite structure materials such as bonded crystals and composite ceramics have been used to mitigate the heat effects.
To achieve constant heating throughout the lasing medium, a tailored longitudinal concentration profile (of active ion dopant) is needed. Lasing media with different concentrations along the length thereof have been used. For example, multiple single crystal segments each having a different concentration are bonded together to form a lasing medium with a stepped dopant concentration profile. However, these lasing media are bonded at interfaces that cross the laser beam axis, resulting in media that are more expensive to fabricate and prone to damage. Alternatively, there are lasing media having a concentration profile that is created by mixing ultra-fine powders of different concentrations along the laser axis of the body. The structure is then sintered to form a dense, optically clear ceramic. However, these media may not exhibit the same superior lasing performance as a pristine single crystal.
What is needed is a method and apparatus that addresses one or more of the deficiencies noted above in fabricating lasing media having a tailored dopant concentration profile.